I dragged the Paul Bear Road-O-Phonic after not having played it for awhile and found my intonation was rather poor.
I swapped to the De Gruchy and had much more success. Why is that?
After a bit investigation, I found that the intonation was out. I know that's a weird concept as you don't physically use the frets, you do use them as a guide. By angling the slide and playing an open 'D' and freting the twelfth fret and looking at the reflection f the fret in the slide, I could see that the perfect spot was a few millimetres high. That can't help.
Tonight, I mustered up the courage and, using a Stanley knife, cut into the wood on the neck side of the wood that rise above the section that is cut away to support the cone. The objective? To have the cone/spider assembly be closer to the neck. I had measured the distance from the nut to the twelfth fret at 29.2 cm. The distance from the bridge was longer. So after a bit of cutting, I had the cone as close to neck as it could be. Measuring the distance again, I found it very close to the nut to twelfth fret distance. I assembled everything and found that the reflection off the slide agreed very closely with the fret position. A quick check with the slide was most gratifying. Almost (probably the same) as the De Gruchy. Now all I have to do is practice a little.
I had tried this procedure some time ago, and had improved the intonation, but not quite right.